Page 35 - Climate Control News Magazine July 2021

Product Focus
Figure 1: Oscillations and vibrations from systems and units are transmitted into the surrounding area.
Figure 2: This is known as the single mass oscillator model.
HVAC system or assemblies placed on them cre- ate, from a physical point of view, an oscillatory system (Figure 2). This is described using the sin- gle mass oscillator model and possesses a reso- nance or natural frequency.
This characteristic vibration frequency is de- rived from the mass m of the system and the dy- namic stiffness c of the elastic bearing.
The oscillatory system is excited by an exter- nal stimulus, e.g. the coolant circuit compressor, and starts to vibrate. The model refers to this as the exciting force. When assessing the isolating effect of a single mass oscillator, the frequency ratio between the excitation frequency of the external stimulus (fe) and the natural frequency of the vibrating system (fo) has to be considered.
At a frequency ratio of
system will vibrate to the same extent specified by the exciting force. Vibrations are transmitted directly into the surroundings. With a frequency ratio of , the excitation frequency corre- sponds to the natural frequency. At this reso- nance point the excitation vibration significant- ly strengthens the natural vibration response, resulting in very large vi-
bration amplitudes.
Steady state operation at this working point must be avoided, as in extreme cas- es damage of the system can happen. This occurs when less expensive rubber pads are placed under- neath the air conditioning units.
Rubber bearings that are just a few millimetres thick and generally extremely stiff provide a very low-cost, simple decoupling solution. Configuring the technical
layout of these products is very difficult, because manufacturers rarely releases any useful data. There is a risk, due to the extreme stiffness of the rubber bearing, that these products have a natu- ral frequency that is very close to the working point of the compressor.
If a frequency ratio of is reached, vi- bration isolation starts to take effect. Here, due to the natural vibration of the oscillatory mass- spring system, the mass forces act against the exciting forces caused by the vibrations induced by the external stimulation.
The vibrating machine mass offsets the stimu- lation caused by the compressor. The effect of the vibration excitation on the surrounding area is therefore considerably lower. As the frequency
“IF A HVAC UNIT IS NOT DECOUPLED, THE OPERATIONAL VIBRATIONS ARE TRANSMITTED INTO THE FOUNDATION.”
ratio increases, so too does the effectiveness of the vibration isolation.
Precise knowledge of the natural frequency of the elastic bearing is very important. Serious providers of elastic bearing elements always of- fer the option of a rapid calculation of the natural frequency at any given machine weight.
This can be done using diagrams in product catalogues or with apps that are made availa- ble online. All the user must know are the mass of the machine and the number of bear- ing points.
In HVAC systems in which the refrigerant circuit compressor is running at a fixed rota- tional speed, the steady state rotation frequen- cy of the compressor should be a factor of 3 to 4 times higher than the natural frequency of the elastic bearing.
, the decoupled
CLIMATE CONTROL NEWS
JULY 2021
35